Extruded solid insulation based on a polyethylene, or a crosslinked polyethylene (XLPE), has for almost 40 years been used for AC transmission and distribution cable insulation. Therefore the possibility of the use of XLPE for DC cable insulation has been under investigation for many years. Cables with such insulations for DC transmission have no restrictions on circuit length and they also have a potential for being operated at higher temperatures, thus offering a possibility to increase the transmission load.
It is, however, well known that this type of XLPE composition exhibits a strong tendency to form space charges under DC electric fields, thus making it unsuitable in insulation systems for DC cables. However, it is also known that extended degassing, i.e., subjecting the crosslinked cable insulation to high temperatures for long periods of time, will result in a decreased tendency to space charge accumulation under DC voltage stress. It is generally believed that the heat treatment removes the peroxide decomposition products, such as acetophenone and cumyl alcohol, from the insulation whereby the space charge accumulation is reduced. Degassing is, however, a time-consuming batch-process comparable with impregnation of paper insulations and thus as costly. Therefore it is advantageous if the need for degassing is removed.
EP 2093774 A1 provides a method for producing an insulated electric high voltage DC cable or a high voltage DC termination or joint with a polymer based, preferably polyethylene, electrical insulation system suitable for use in DC transmission and distribution networks carried out in a manner such that there is no need for any lengthy time consuming batch-treatment (e.g. heat treatment) of the cable to ensure stable and consistent dielectric properties and a high and consistent electric strength of the cable insulation. The resulting cable insulation further exhibits a low tendency to space charge accumulation, a high DC breakdown strength, and a high impulse strength and high insulation resistance.
The method comprises subjecting the polymer based insulation system for a heat treatment procedure while the outer surface of the polymer based insulation system is covered by a cover impermeable to at least one substance present in the polymer based insulation system in a non-homogenous distribution, thereby equalizing the concentration of the at least one substance in the polymer based insulation system. The method may be performed for a DC cable subsequent to subjecting the DC cable for a heat treatment while the outer surface of the extruded polymer based insulation system is not covered by any impermeable cover to thereby remove another substance, e.g. methane, present in the polymer based insulation system after crosslinking. The at least one substance may include a rest or byproduct from crosslinking, preferably a peroxide decomposition product, and/or an additive, e.g. an antioxidant.
The heat treatment procedure is performed at a temperature of between 50 and 120° C., and most preferably between 70 and 90° C., and is relatively fast.